AD8027ART-R2 datasheet(22/25 Pages) AD

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AD8027/AD8028

Rev. C | Page 21 of 24

APPLICATIONS

USING THE SELECT PIN

The AD8027/AD8028’s unique SELECT pin has two functions:

• The power-down function places the AD8027/AD8028 into

low power consumption mode. In power-down mode, the

amplifiers draw 450 μA (typical) of supply current.

• The second function, as mentioned in the Theory of

Operation section, shifts the crossover point (where the

NPN/PNP input differential pairs transition from one to the

other) closer to either the positive supply rail or the negative

supply rail. This selectable crossover point allows the user to

minimize distortion based on the input signal and environ-

ment. The default state is 1.2 V from the positive power

supply, with the SELECT pin left floating or in three-state.

Table 6 lists the SELECT pin’s required voltages and modes.

Table 6. SELECT Pin Mode Control

SELECT Pin Voltage (V)

Mode

VS = ±5 V

VS = +5 V

VS = +3 V

Disable

−5 to −4.2

0 to 0.8

Crossover Referenced

–1.2 V to Positive

Supply

−4.2 to

−3.3

0.8 to 1.7

Crossover Referenced

+1.2 V to Negative

Supply

−3.3 to +5

1.7 to 5.0

1.7 to 3.0

When the input stage transitions from one input differential

pair to the other, there is virtually no noticeable change in the

output waveform.

The disable time of the AD8027/AD8028 amplifiers is load-

dependent. Typical data is presented in Table 7. See Figure 52

and Figure 53 for the actual switching measurements.

Table 7. DISABLE Switching Speeds

Supply Voltages (RL = 1 kΩ)

Time

±5 V

+5 V

+3 V

tON

45 ns

50 ns

tOFF

980 ns

1100 ns

1150 ns

DRIVING A 16-BIT ADC

With the adjustable crossover distortion selection point and low

noise, the AD8028 is an ideal amplifier for driving or buffering

input signals into high resolution ADCs such as the AD7767, a

16-bit, 1 LSB INL, 1 MSPS differential ADC. Figure 60 shows

the typical schematic for driving the ADC. The AD8028 driving

the AD7677 offers performance close to non-rail-to-rail

amplifiers and avoids the need for an additional supply other

than the single 5 V supply already used by the ADC.

In this application, the SELECT pins are biased to avoid the

crossover region of the AD8028 for low distortion operation.

Summary test data for the schematic shown in Figure 60 is

listed in Table 8.

+5V

–

AD8028

ANALOG INPUT +

INPUT RANGE

(0.15V TO 2.65V)

SELECT

(OPEN)

SELECT

(OPEN)

–

ANALOG INPUT –

AD7677

+5V

16 BITS

2.7nF

4MHz LPF

2.7nF

0.1

μF

0.1

μF

AD8028

03327-A-059

Figure 60. Unity Gain Differential Drive

Table 8. ADC Driver Performance, fC = 100 kHz,

VOUT = 4.7 V p-p

Parameter

Measurement

Second Harmonic Distortion

–105 dB

Third Harmonic Distortion

–102 dB

THD

–102 dB

SFDR

+105 dBc

As shown in Figure 61, the AD8028 and AD7677 combination

offers excellent integral nonlinearity (INL).

CODE

16384

32768

49152

65536

–1.0

–0.5

0.5

1.0

03327-A-060

Figure 61. Integral Nonlinearity

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